The simplest way to direct all mail to a subfolder would be to create a
".courier" file in your home directory with one line:

./Maildir/.ABC/

Yes, I already thought about this way. But is this safe? As far as I
understand, you should normally not access the subfolder directories
(like .ABC) directly. For example, if you want to add a folder DEF as a
subfolder of ABC, you have to use

maildirmake -f ABC.DEF ~/Maildir

instead of

maildirmake -f DEF ~/Maildir/.ABC,

i.e., you use ~/Maildir as the mail directory, not ABC's directory
~/Maildir.ABC.

What does the Courier MTA or Maildrop do when delivering an e-mail? Does it
just put the mail in a file in the "new" directory? Then your approach would
be safe. Or does it also update certain files located at the top level of
the maildir?*) Then your approach would be errornous since the MTA/Maildrop
would try to update files in ~/Maildir/.ABC while they in fact reside in
~/Maildir.

*) Courier IMAP does so, for example. It creates courierimapsubscribed and
courierimapuiddb.

[...]

The background of this whole thing is that I want to have IMAP folders
like Inbox, Sent and Trash which reside on the same hierarchy level.

Then "modify" your IMAP account configuration in Kmail and set the
"Prefix to folders" setting to "INBOX".

But then I'm not able to access the top level INBOX, right? So I still have
to deliver mail to a subfolder of INBOX.

[...]

If I just let Courier direct mail to ~/Maildir and access ~/Maildir with
my e-mail client (KMail) via IMAP, I have a top level folder Inbox and all
other folders I create are located below Inbox. This is, of course,
nonsensical.

I sometimes think the same thing about the noise made over a such a
cosmetic feature. ;)